I don’t follow... Was Stephen filled with grace? Did he need to be sinless to be filled with grace? If Stephen was filled with grace and was not sinless, then being filled with grace does not imply a sinless life.
**? If Stephen was filled with grace and was not sinless, **
Even I am “full of grace” after having said my penance after the Sacrament of Reconciliation. But that doesn’t mean that I am sinless.